1. Field of the Invention
This invention relates to vapor treatment facilities and, in particular, to the application of such facilities in cleaning of petroleum storage tanks.
2. Brief Statement of the Prior Art
Petroleum and its volatile products are stored at bulk terminals, refineries and the like in floating or fixed roof tanks. In floating roof storage tanks, the tank roof floats on the liquid contained within the tank, thereby greatly reducing the volume of vapor over the liquid in the tank. Although these tanks minimize the volume of vapors which are discharged to the atmosphere during normal operations, substantial quantities of vapors are discharged to the environment when the tank is completely emptied, as the floating roof descends to a minimum height which is approximately 6 feet above the bottom of the tank. The tanks are cleaned periodically, and it is necessary to flush the mixture of air and hydrocarbon gases within the tank with fresh air, until the hydrocarbon content of the mixture is sufficiently low to permit workmen to enter the tank for cleaning operations.
During storage of volatile petroleum distillate products such as gasoline, the mixture or air and gasoline vapors within the tank can contain up to about 35 percent hydrocarbons. Additionally, unrefined petroleum crude products such as untreated sour hydrocarbons, can contain sulfur compounds such as mercaptans, hydrogen sulphide, sulfur dioxide and the like. The discharge of the hydrocarbons or the sulfur compounds into the atmosphere during flushing of the tanks is of increasing environmental concern. In some areas of the country, notably Southern California, existing or threatened legislation will prohibit the discharge of this vapor/air mixture directly into the atmosphere.
Various refrigeration units have been proposed to recover vapors from underground gasoline storage tanks at gasoline service stations, or other low volume sources of hydrocarbons, prior to filling of these tanks. These tanks, however, are very small in comparison to the large bulk storage tanks at oil terminals and refineries, and the prior units are far undersized for use in treating the air/vapor mixture from a bulk oil storage tank. Additionally, service station sites are readily accessible and have electrical power supplies which accommodate a portable refrigeration unit, conditions which are seldom present at bulk petroleum storage tank sites.